The return-to-zero (RZ) modulation format is important for long haul and ultra-long haul high bit-rate optical fiber communication systems. Here, reference to high-bit rate systems is intended to mean lightwave systems operating at a bit-rate of at least 10 Gb/s, and preferably 40 Gb/s. The RZ format implies that the optical pulses in the pulse stream return to a zero amplitude level at the beginning and end of each pulse. This is especially valuable in long haul and ultra-long haul optical fiber transmission systems where the system performance is limited by amplified spontaneous emission (ASE) noise and fiber nonlinearity. A chirp-free optical pulse generator is crucial in the design of an RZ transmitter. The most commonly used technique to generate a chirp-free RZ pulse stream employs a continuous wave (CW) laser and a sinusoidally driven Mach-Zehnder modulator (MZM), typically a LiNbO3 modulator. See U.S. Pat. No. 5,477,375, “Optical soliton generator” to Korotky, et al. Recently we have demonstrated an alternative technique employing phase modulation and a delay interferometer to produce chirp-free optical pulses. See U.S. patent application Ser. No. 10/144,477, X. Wei, J. Leuthold, and L. Zhang filed on May 13, 2002), which is incorporated herein by reference. Similar to the MZM approach, the delay interferometer approach is also interference-based.